New catalytic route to functionalized vinylboronates

Article abstract of DOI:10.1039/b414644a

Vinylsubstituted boronates i.e. vinyldioxaborolane and vinyldioxaborinane react regioselectively with olefins in the presence of RuHCl(CO)(PCy ₃)₂ with the formation of functionalized vinylboron derivatives. The reaction opens a new catalytic route for preparation of organoboranes. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b414644a

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New catalytic route to functionalized vinylboronates{
Bogdan Marciniec,* Magdalena Jankowska and Cezary Pietraszuk
Received (in Cambridge, UK) 23rd September 2004, Accepted 2nd November 2004
First published as an Advance Article on the web 14th December 2004
DOI: 10.1039/b414644a
Fivefold excess of olefin was used in order to avoid the homo-
Vinylsubstituted boronates i.e. vinyldioxaborolane and vinyl-
dioxaborinane react regioselectively with olefins in the presence
of RuHCl(CO)(PCy3)2 with the formation of functionalized
vinylboron derivatives. The reaction opens a new catalytic route
for preparation of organoboranes.
coupling of vinylboronate. For styrenes trans-borylation proceeds
stereo- and regioselectively giving E-styryl boronates. When
1-alkenes were used as the reacting partner, the formation of
significant amounts of vinylboronate homo-coupling products
could not be avoided, even at a fivefold excess of an olefin. In
addition, in the presence of ruthenium hydride complexes, the
reaction is accompanied by olefin isomerization, so in this case a
mixture of alkenylboronates (with a high predominance of E-1,2-
alkenylboronates) is formed. The obtained results are compiled in
Table 1.
Functionalized vinylboronates make a class of organoboron
compounds commonly used in organic synthesis since the
boronate moiety can be converted into other functional groups
(aldehyde, halide, amine, alkyl etc.).1,2 Methodologies including
classical stoichiometric routes as well as hydroboration of alkynes
are usually employed to prepare vinylboron reagents.3,4 Recently
new catalytic methods for their synthesis, namely Heck-type
borylation,5 cross-coupling borylation,6 dehydrogenative boryla-
tion7 and cross-metathesis8 of alkenes with vinylboronates, have
been reported.
In order to compare the applicability of trans-borylation and
cross-metathesis for the synthesis of vinylboron derivatives, the
same parent substances were also tested in cross-metathesis in the
presence of Grubbs 1st generation catalysts Cl2(PCy3)2Ru(LCHPh)
(II) [eqn. (2)].
In the last two decades we developed two universal methods for
the synthesis of well-defined molecular compounds with vinylsi-
licon functionality. Both methods i.e. silylative coupling (also
called trans-silylation or silyl group transfer) and cross-metathesis,
are based on catalytic transformations of vinyl-silicon compounds
with olefins and lead to synthesis of respective functionalized vinyl-
silicon reagents.9 While the cross-metathesis (CM) is catalyzed by
well-defined Ru and Mo carbene complexes, the silylative coupling
(SC) takes place in the presence of complexes initiating or
generating M–H and M–Si bonds (where M 5 Ru, Rh, Ir). The
mechanism of SC proved by Wakatsuki10 and by us11 proceeds via
insertion of vinylsilanes into the M–H bond and b-Si transfer to
the metal with elimination of ethylene to generate a M–Si species,
followed by insertion of an alkene into the M–Si bond and b-H
transfer to the metal with elimination of substituted vinylsilane.
We have found that this mode of reactivity, well known for
vinylsilanes, seems to be more general and is exhibited also by
vinylboronates. In this communication we report a new catalytic
transformation of vinylboranes with alkenes which occurs in the
presence of a catalytic amount of RuHCl(CO)(PCy3)2 (I)
according to the non-metallacarbene mechanism, and leads to
the effective formation of vinylboron derivatives12 [eqn. (1)].
ð2Þ
A fivefold excess of olefin was used in order to eliminate the
formation of vinylboronate homo-metathesis products. However,
in these conditions some amounts of olefin homo-metathesis
products cannot be avoided. CM and trans-borylation were found
to give the same boron derivatives [see eqns. (1) and (2)]. In
contrast to unsuccessful trans-borylation, relatively high yield of
cross-metathesis products was obtained for the reaction with
1-alkenes, which is in good agreement with earlier results.8 The
results obtained are given in Table 1. Non-isolated products were
identified on the basis of mass spectra.
To distinguish between the metallacarbene mechanism of
metathesis and the non-metallacarbene mechanism of trans-
borylation, the reactions of styrene-d8 with vinyldioxaborolane
catalyzed by I or II have been studied.13 In the case of a non-
metallacarbene mechanism the formation of borylstyrene-d7 and
ethylene-d1 is to be expected (Scheme 1). In contrast, the
metallacarbene mechanism should afford borylstyrene-d6 and
ethylene-d2.
ð1Þ
Analysis of the reaction mixture after 0.5 h, i.e. in the initial
stage of reaction when the conversion did not exceed 10%, revealed
only the formation of borylstyrene-d7 while d6 olefin was not
detected. Therefore, the metallacarbene mechanism in the condi-
tions of trans-borylation reactions12 can be excluded. In contrast, a
similar experiment clearly proved the metallacarbene mechanism
{ Electronic supplementary information (ESI) available: synthesis and
spectroscopic data for III and styrylboronates. See http://www.rsc.org/
suppdata/cc/b4/b414644a/
*marcinb@amu.edu.pl
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Table 1 Trans-borylation (TB) and cross-metathesis (CM) of vinylboronates with selected olefins
Trans-borylation
Yield (E/Z) [%]
Cross-metathesis
Homo-metathesis by-products
R 5
Yield (E/Z) [%]
Yield [%]
C6H5
BO2C2H4
91a(E)
55 (6/1)
59a
—
11b
80c
C6H5
80a (E)
90 (E)
95 (E)
38d
85a (E)
85 (E)
90 (E)
75 (25/1)
5b
0
C6H4–Me–4
C6H4–Cl–4
C6H13
0
15b,,8c
pyrrolid–2–one
OEt
OPr
BO2C3H6
b
10 (E)
60 (E)
80 (E)
80 (8/1)
c
0
0
0
—
0
0
0
60c
a
d
Isolated yield. Homo-metathesis of olefin. Homo-metathesis of vinyl boronate.
5
isomers formed, reaction accompanied by
vinyldioxaborinane homo-coupling. Reaction conditions: trans-borylation: benzene, 80 uC, 3 h, [vinylboronate]:[olefin] 5 1:5. Cross-metathesis:
CH2Cl2, 40 uC, 3 h, [vinylboronate]:[olefin] 5 1:1
Presented results allow us to propose a reasonable mechanism
for the reaction of vinylboronates with styrene (alkene) (Scheme 2).
A mechanistic scheme of this new type of vinylboronate
conversion, studied for the reaction with styrene, involves the
migratory insertion of styrene, a representative alkene (or
vinylboronate in the case of homo-coupling) into a Ru–B bond
followed by b-H elimination to give E-phenyl(boryl)ethene and
insertion of vinylboronate into a Ru–H bond followed by b-B
elimination of ethylene (Scheme 2). Dissociation of phosphine is
postulated to generate the active catalyst.16 The proposed
mechanism is analogous to that proved for silylative coupling.11b
In conclusion, borylation of non-isomerising olefins with
vinylsubstituted boronates, i.e. vinyldioxaborolane and vinyldiox-
aborinane, in the presence of catalysts containing an Ru–H bond
opens a new, effective catalytic route to functionalized vinyl
boronates.
Scheme 1 Trans-borylation and cross-metathesis of styrene-d8 with
vinyldioxborolane.
of the reaction carried out in the presence of the Grubbs catalyst
(II). In these circumstances GC/MS analysis (performed after
15 min of the reaction progress) revealed the exclusive formation
of borylstyrene-d6.
In order to provide evidence for the insertion of olefin into Ru-B
bond, stable Ru(BO2C6H4)Cl(CO)(PCy3)2 (III) was prepared14
and tested in reaction with a twofold excess of styrene. The
reaction yielded styrylboronates (identified by NMR spectroscopy
and GC-MS) and hydride complex RuHCl(CO)(PCy3)2 (identified
This work was supported by grant No. 3 T09A 145 26 from the
State Committee for Scientific Research (Poland)
1
by H NMR spectroscopy) according to the proposed equation
[eqn. (3)].{
ð3Þ
In another experiment, a benzene solution of complex I was
treated with a twofold excess of vinylcatecholborane. Analysis of
the reaction mixture by 1H NMR demonstrated formation of
ethylene [eqn. (4)], however boryl complex was not isolated.
ð4Þ
While insertion of an olefin into an M–B bond is a known
process,15 formation of Ru–B bond via migration of a boryl group
from a position b- to the metal [as proposed in eqn. (4)] has not
been, to the best of our knowledge, reported in the literature.
Scheme 2 Proposed mechanistic scheme for trans-borylation.
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Bogdan Marciniec,* Magdalena Jankowska and Cezary Pietraszuk
Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6,
Organometallics, 1993, 12, 975; (c) J. M. Brown and G. C. Lloyd-Jones,
J. Am. Chem. Soc., 1994, 116, 866; (d) D. H. Motry, A. G. Brazil and
M. R. Smith, III, J. Am. Chem. Soc., 1997, 119, 2743; (e) M. Murata,
S. Watanabe and Y. Masuda, Tetrahedron Lett., 1999, 40, 2585; (f)
M. Murata, K. Kawakita, T. Asana, S. Watanabe and Y. Masuda, Bull.
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´
60-780, Poznan, Poland. E-mail: marcinb@amu.edu.pl;
Fax: (+48) 61 8291 508; Tel: (+48) 61 8291 509
Notes and references
8 C. Morril and R. H. Grubbs, J. Org. Chem., 2003, 68, 6031.
9 For recent reviews on silylative coupling and cross-metathesis of
vinylsilicon functionality see B. Marciniec and C. Pietraszuk, Curr. Org.
Chem., 2003, 7, 691.
10 Y. Wakatsuki, H. Yamazaki, M. Nakano and Y. Yamamoto, J. Chem.
Soc., Chem. Commun., 1991, 703.
11 (a) B. Marciniec and C. Pietraszuk, J. Chem. Soc., Chem. Commun.,
1995, 2003; (b) B. Marciniec and C. Pietraszuk, Organometallics, 1997,
16, 4320.
12 Representative procedure of trans-borylation: An oven dried Schlenk flask
equipped with a condenser and a magnetic stirring bar was charged
under argon with benzene (3 mL), dodecane (internal standard),
vinyldioxaborinane (0.05 mL, 0.45 mmol) and styrene (0.27 mL,
2.59 mmol). The reaction mixture was heated to 80uC and the
ruthenium hydride complex RuHCl(CO)(PCy3)2 (0.0032 g,
0.0044 mmol) was added and the reaction was carried out for 3 h.
13 For similar experiment see: ref. 7a.
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14 Analogous procedure to that described for Ru(BO2C6H4)Cl(CO)(PPh3)2
was used; G. J. Irvine, W. R. Roper and L. J. Wright, Organometallics,
1997, 16, 2291.
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16 It was found that a small (threefold in relation to catalyst) excess of free
phosphine strongly retards the process.
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